Among the infrared detectors that have been put to practical use, those detecting infrared rays in the middle- and far-infrared wavelength ranges (for example, a wavelength band ranging from 3 μm to 12 μm) include: infrared detectors made of a semiconductor of HgCdTe, which is a group II-VI compound; quantum well infrared photo-detectors (QWIPs) made of a superlattice of a group III-V compound semiconductor (GaAs/AlGaAs); and quantum dot infrared photo-detectors (QDIPs) made of a quantum dot.